The present invention relates to communication systems and in particular to networks with communication compression units.
Many communication links allow compression of the voice signals which they transfer, in order to increase the throughput of the link. At both ends of these links, a compression/decompression unit compresses the signals entering the link and decompresses the signals coming out of the link. In some cases, voice signals of a telephone call pass over a path including a plurality of links on which the signals are compressed. The compression of the signals generally reduces the quality of the voice signals and the quality reduction is a function of the number of times the signals are compressed and decompressed. Therefore a method has been devised to eliminate multiple compressions. In this method, when a decompression unit is aware of an additional compression unit through which the voice signals are to pass along their path, the signals are left in their compressed form. Such a method has been described for example in Applicant co-pending patent application U.S. Ser. No. 09/465,456 filed Dec. 17, 1999 which is hereby incorporated by reference.
The compression/decompression units replace a small percentage of the voice signals during compression with flag bits which notify other units that compression has been performed. These flag bits will be referred to hereinafter as xe2x80x9ccompression flagsxe2x80x9d. A compression/decompression unit receiving these flag bits on the voice signals in a first direction, preferably refrains from decompressing the voice signals it transmits in the downstream direction. Instead, the compression/decompression unit passes the signals in the downstream direction in their compressed form, as there is an additional compression/decompression unit along the path of the signals for their decompression. Compression flag bits are also transmitted with the signals transmitted in their compressed form. Such a mode of operation is known in the art as xe2x80x9cend-to-end compressionxe2x80x9d. If while transmitting compressed signals, the compression flag bits are not received in their entirety, the compression/decompression units revert to the mode wherein the signals are decompressed and transmitted in their decompressed form.
Switching between a state in which the signals are transmitted in the compressed form and a state in which the signals are transmitted in a decompressed form, may involve a short period in which the voice signals are not provided properly to the users. Many such state transitions during a telephone conversation may be very annoying.
An aspect of some embodiments of the present invention relates to a compression/decompression unit which does not transmit the signals in their compressed form in all cases in which such transmission is possible, i.e., the received signals from the downstream direction include compression flags. Rather, when it is estimated that the possibility to transmit the signals in the compressed form will be only for a short period, the signals are not transmitted in their compressed form. Optionally, the transmission of the signals in the compressed form is prevented when more than a predetermined number of state transitions between transmission of compressed and decompressed signals occurred within recent period of a pre-defined length of time. Such state transitions may occur, for example, when voice signals are transmitted in a conference call.
In some embodiments of the invention, when a decompression unit receives compression flags while transmitting decompressed signals, it determines whether to switch to a state in which it transmits compressed signals based on the time which passed since the most recent time it operated in the compression state. Optionally, only if a predetermined period of time lapsed since the last time the decompression unit operated in the compression state is the unit allowed to return to the compression state, pending the receipt of the appropriate compression flags.
In some embodiments of the invention, the compression flag bits transmitted by compression and decompression units include indication of the unit transmitting the compression flags. The determination of whether to switch to compression state responsive to reception of compression flags is optionally performed responsive to actions related to the specific unit from which the compression flags were received. Thus, when a conference call is terminated and a different call begins through a different compression and decompression unit, the unit receiving the compression flags may switch immediately to the compression state.
According to the present invention there is provided a method of transmitting signals along a communication path, by a first compression/decompression unit, which method comprises:
receiving first signals from a first direction along the path;
determining based on the received first signals whether a second decompression unit is located along the path in the first direction;
receiving second signals from a second direction;
in the case it is determined that a second decompression unit is located along the path in the first direction, selecting a compressed or decompressed format for transmission of the second signals responsive to at least one additional parameter, such that the determination that a second decompression unit is located along the path in the first direction does not always bring about selecting the compressed format; and
transmitting the second signals in the selected format.
According to a preferred embodiment of the invention the at least one additional parameter comprises a pre-defined period of time that lapsed since a previous occurrence in which signals were transmitted in the second direction by said first compression/decompression unit in their compressed format. Alternatively or additionally, the at least one additional parameter comprises a number of changes in the transmission format during a recent period. Still alternatively or additionally, the at least one additional parameter comprises a period of time passing since a previous time the first signals were determined to indicate that a second decompression unit is located along the path in the first direction.
In accordance with another preferred embodiment of the invention, the method provided further comprises determining an identification of the second decompression unit located along the path in the first direction, e.g. identification that is derived from the first signals received. According to this embodiment the at least one additional parameter may be either one of the parameters described above or any combination thereof. Alternatively or additionally, the at least one additional parameter may comprise information regarding the number of times a change occurred within a recent period of time, in the determination of whether a second decompression unit having the same identification as that currently received is located along the path in the first direction.
By yet another embodiment of the invention, the selection of a format for transmitting the second signals comprises selecting a decompression format even if an indication that a second decompression unit is located along the path in the first direction is received, when it is estimated that the communication path is leading signals of a conference call. Alternatively or additionally, the selection a format for transmitting the second signals comprises selecting a decompression format even if the received first signals were determined to indicate that a second decompression unit is located along the path in the first direction, if there is a high probability that the content of at least some of the first signals that are expected to be received in the near future will prevent the selection of the compressed format.
By another embodiment of the invention, the result achieved in determination step of the method provided, is affected by the fact whether the first signals implanted by the second compression/decompression unit are received intact.
According to another aspect of the invention, there is provided a compression/decompression unit which comprises:
a first input interface for receiving first signals from a first direction along a communication path;
a second input interface for receiving second signals from a second direction along the communication path;
a selection unit which selects a compressed or decompressed format for transmission of the second signals received, following a determination whether a second decompression unit is operative in the first direction along the communication path, and upon considering at least one additional parameter, such that at least in some cases in which a second decompression unit is determined to be operative in the first direction along the communication path, a decompressed format is selected; and
an output interface through which the second signals received through the second input interface are transmitted in the selected format.
Once the decompressed format is selected by the selection unit, the compression/decompression unit should be operative to decompress the second signals received.
Alternatively or additionally, the selection unit selects the decompressed format even when the content of currently received first signals allows selection of the compressed format, if there is a high probability that the content of at least some of the first signals received in the near future will not allow selection of the compressed format.
Still alternatively or additionally, the selection unit is capable of selecting the decompressed format even when the content of the currently received first signals allows selection of the compressed format, if recently received first signals did not allow selection of the compressed format. Similarly, mutatis mutandis, the selection unit selects the decompressed format even when the content of the currently received first signals allows selection of the compressed format, if recently received first signals caused the selection unit to change its selection from the compressed format to the decompressed format.
According to yet another aspect of the invention there is provided a communication transmission system which comprises:
a plurality of compression links which pass signals in a compressed format; and
a plurality of compression/decompression units located along said plurality of compression links, each of which is operative to determine whether to transmit signals received from respective compression links in a compressed or decompressed format, responsive to a determination of whether they currently belong to a communication connection which includes at least one additional unit in the direction of transmission and responsive to at least one additional parameter, such that at least in some cases in which the connection includes at least one additional unit in the direction of transmission, the signals are transmitted in the decompressed format.
In a preferred embodiment of the invention, the at least one additional parameter comprises a probability of the current communication connection belonging to a conference call.